1. Field of the Invention
The invention concerns a magnetic resonance apparatus having a magnetic field generation device for generating a magnetic field.
2. Description of the Prior Art
As is generally known, a magnetic resonance apparatus serves for acquisition of slice images of an examination subject using various magnetic fields to which the patient is exposed. A temporally-constant magnetic field (the basic magnetic field) is generated by a magnetic field generation device for polarization of the atomic nuclei in the body to be examined. The magnetic field generation device is typically executed as an axially superconducting magnet. This magnetic field is continuously generated because an alternating activation and deactivation of the superconducting magnet is not operationally possible; it is thus permanently on. Gradient fields directed in three spatial directions for spatial coding are generated by a typically cylindrical gradient coil while an alternating magnetic field for excitation of the atomic nuclei is generated by a radio-frequency or measurement coil. The radio-frequency alternating field that arises from the processing nuclear moment is also detected and is supplied as an image signal to the downstream image processing.
While the gradient and magnetic fields are generated only for examination of the subject, as stated the magnetic field generated by the magnetic field generation device (in the form of the superconducting basic field magnet) is continuously present. This magnetic field exhibits a field strength of multiple T, typically 0.1-3T.
As is generally known, the permanent, temporally constant magnetic field is present not only inside the magnetic resonance apparatus (typically a cylindrical bore) into which the patient is inserted for image acquisition, but also its stray field extends considerably far in regions outside of the magnetic field generation device, or around it. If a ferromagnetic object is located near to the magnetic field generation device in the region of the still-effective magnetic field (stray field), an interaction of the ferromagnetic object with the magnetic field occurs, causing the object to be attracted by the magnetic field and pulled onto or into the magnetic field generation device. Such an object then adheres to the housing of the magnetic field generation device or inside the bore at the respective wall and inevitably influences the generated magnetic field.
Typical magnetic resonance apparatuses are located in sealed cabins in which no loose ferromagnetic objects are present. Nevertheless, it frequently occurs that persons enter this room and bring such objects with them. Such objects can be, for example, scissors, a ball-point pen, a watch, coins etc. (thus arbitrary objects that are frequently carried by persons who work on or with magnetic resonance apparatuses). Although these persons are obligated to deposit such objects outside of the room in order to avoid these objects being attracted to the magnetic resonance apparatus due to interaction, this requirement is frequently not respected. As a result, it is relatively often that an object is attracted by the magnetic field and strikes against the magnetic resonance apparatus. Insofar as the user recognizes this, he or she typically immediately removes the object. Nevertheless, it can occur that such an impact is not detected; the object then adheres on the magnetic resonance apparatus and disadvantageously influences the magnetic field and consequently the image acquisition. Such objects can also cause damage to the magnetic resonance apparatus depending on where the object strikes, and its momentum. The magnetic field is still very strong in the region outside of the magnetic resonance apparatus, such that even small objects strike with large momentum on the magnetic resonance apparatus. Such damage can even lead to an actual destruction of the magnetic resonance apparatus, which then possibly fails and would have to be serviced. The person causing such a failure often cannot be traced by the device operator because, as stated, in such a case a person who has accidentally taken such an object into the room with him or her usually immediately removes the attracted object after the damage has been done.